Our experience on methotrexate pharmacogenetics is related to the evaluation of the most common functional variants in ATIC, inosine triphosphate-pyrophosphatase (ITPA) and solute carrier family 19 member 1 (SLC19A1): we selected these candidate variants on the basis of recent studies evaluating comprehensively genetic polymorphisms in the methotrexate pharmacokinetic and pharmacodynamic pathways in patients with JIA and rheumatoid arthritis (RAs; Dervieux et al., 2011; Hinks et al., 2011; Owen et al., 2012). The gene discussed is ITPA; the disease is rheumatoid arthritis.